The Twilight Experiment

The aureole of Venus, showing up during the egress in 2004, as imaged by a coronagraph built by A. and S. Rondi

Three extra-large suitcases in a high speed train from Nice to Paris, France, have attracted some attention, especially at the end of the travel in the passenger crowd pressed to leave the cars. It happened on May 14. Later that day, they were transported to the observatory in Meudon, opened and inspected. The six precious instruments had done a good travel – the first leg of a worldwide adventure.

This is just a recent step of the “Venus Twilight Experiment”, born from the joint effort of Thomas Widemann (Paris Observatory), the author of this note – later supported by a network of several researchers.

Basically, the idea is to get the best possible data on the aureole from a homogeneous set of identical instruments, placed in the region of visibility of the transit.

The core of the coronagraph: on the left, the cone for occulting the Sun.

By hiding the light of the Sun behind an occulting disk, the contrast obtained on the aureole is the best possible. Also, by accurately placing appropriate stops in the optical path, light scattered by the telescope objective can be considerably reduced. These are the principle of the classic “coronagraph”, the instrument fist conceived and built by B. Lyot for enhancing the visibility of solar prominences.

In our case, we modified the design for taking into account that we are not interested in imaging prominences all around the solar disk, but only a small area centered on the contact points between the Sun and Venus, at the ingress and egress. As a consequence, the optical axis is not at the Sun’s center, but on its limb. Also, the magnification is such that a convenient image scale of the Venus aureole can be obtained. André and Sylvain Rondi, two exceptional French amateurs, helped in the design of the instrument.

This concept was just on paper last autumn, when it became clear that the effort required to design from scratch a campaign for the aureole was well worth the effort. In fact, the scientific interest for the transit – and for the aureole in particular – began to grow attracting much attention. Why? First, we already knew that it is possible to better study the mesosphere of Venus itself (80-110 km above the surface), by a careful characterization of the refraction producing the aureole. The principle was tested by the data collected in 2004, but for that transit no systematic campaign had been organized. The 2012 event is our last opportunity for a targeted, well-planned experiment. Also, in the meantime the interest for the transit of telluric exoplanets in front of their stars has grown, thanks to the discovery of several candidate planets in this category. Is this motivation not sufficient for proposing to study the transit of Venus as the nearby prototype of an exoplanet, much easier to exploit?

Several researchers began focusing their efforts on existing facilities, large solar telescopes in particular. They will certainly offer high-quality data of the best possible resolution, but with the constraints dictated by being built for other kinds of observations.

One of our coronagraphs (the longest tube) set up in Australia, ready to observe (hoping that the clouds will go away!)

Having a dedicated telescope, to place at strategic sites, presented several advantages in itself. These motivations were strong enough to imagine the construction not of one, but several coronagraphs. We thus asked financial support for a small set, and in the end we have built 9 of them, at the workshops of the Observatoire de la Côte d’Azur, in Nice (one of the main supporting institutes along with LESIA, Obs. de Paris, Laboratoire Lagrange, Observatoire de la Côte d’Azur, Paris VII Univ.).

With time passing and the transit approaching, the interest around the experiment has grown beyond expectations. Several observers offered their expertise or proposed complementary observations to be done at different facilities, mainly solar telescopes but also from space (the Picard satellite). Different sites declared their interest in providing logistic support for the observation. The Venus Twilight Experiment was born!

The sites of the Venus Twilight Experiment. In blue, the sites where the coronagraphs are situated. In red, other talescopes participating to the campaign.

We thus identified the sites where the instrument should go, over a wide area, each one with a filter chosen among 4 possible wave bands. This way, we hope to have color information and good data from at least a part of the sites, in case the others are clouded out.

Right now, all the 9 coronagraph are at the chosen sites, and observers are practicing with the procedures of observation. It is the first time that such instruments are used, and several tricky details must be carefully mastered for an appropriate imaging.

I am at Lowell observatory, and I’ll let you know how things will go!

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